A problem with diesel engine exhaust gas is that nitrogen oxide (NOx) and fine particles mainly consisting of carbon (hereinafter, which may be referred to as “PM”) are contained in the exhaust gas, and are a cause of environmental pollution.
A typical method for removing PM is to trap the PM by arranging a diesel particulate filter (DPF) that comprises a porous ceramic in an exhaust gas flow channel.
The PM accumulates in the DPF, and the trapped PM is intermittently or continuously subjected to a combustion treatment to be removed therefrom. Consequently, the DPF can be returned to the state it was in before the PM trapping.
Examples of generally employed methods for this DPF regeneration treatment include a method for combusting PM by forced external heating with an electric heater, a burner or the like, and a method in which an oxidation catalyst is arranged on the side nearer to the engine than the DPF, so that the NO contained in the exhaust gas is converted by the oxidation catalyst into NO2, and the PM is combusted by the oxidative power of the NO2.
However, using an electric heater or a burner requires installation of an external power source. This means that an additional mechanism for maintaining and operating such equipment is necessary, which makes the exhaust gas purification system itself more complicated.
In addition, the oxidation catalyst suffers from various problems, such as the exhaust gas temperature not being high enough for catalytic activity to be sufficiently exhibited, and the NO required for PM combustion not being contained in the exhaust gas unless the system is operated under certain conditions.
As a more preferred DPF regeneration treatment method, a method is being investigated in which the DPF itself is made to support the catalyst, to thereby lower the PM combustion starting temperature due to the catalytic action of the DPF, and then carry out PM combustion.
The ultimate objective which is the most desirable way is a method which achieves continuous PM combustion at the exhaust gas temperature.
Currently, a Pt catalyst metal supported on alumina or the like having a high specific surface area is used as an oxidation catalyst (PM combustion catalyst) for combustion and removal of PM trapped by a DPF.
However, at the exhaust gas temperature level, the catalytic effect of Pt for PM combustion is low. Therefore, continuous PM combustion utilizing the heat of the exhaust gas is considered difficult.
Specifically, external forced heating means is required.
In addition, another problem is that Pt is expensive and causes costs to increase.
Moreover, it can also be assumed that in a PM combustion catalyst, the catalyst temperature may rapidly increase from the heat generated during PM combustion.
Accordingly, there is a need to develop catalyst substances having deterioration (thermal degradation) in catalyst performance as low as possible when exposed to a thermal history at a high temperature.
Patent Documents 1 to 3 disclose, as an oxidation catalyst formed from a base material of a ceria composite oxide free from precious metal elements such as Pt, a mixture including Ce and Bi, and optionally a transition metal element.
A composite oxide formed from Ce and Bi alone, or formed from Ce, Bi, and a transition metal element, releases free Bi when exposed to a high temperature of around 800° C. for a long period of time, because the melting point of the element Bi is low. This causes catalytic activity to deteriorate.
Consequently, there is a need for further improvement regarding heat resistance.